I buy strings for several guitars from MF in large lots, so I don't run out when I need a fresh set. I have used these in the past with good satisfaction but on this last order there was significant corrosion on the unwound strings in multiple packs. One B string had a dozen corroded spots, 3 of which were heavy and exceeded 1/4" in length. I used two packs to get one marginal set on the guitar. This is the first time have experienced this, but I won't use them again.